I. Field of the Invention
This invention relates generally to core-shell electrocatalyst supports. In particular, the present invention relates to core-shell electrocatalyst particles, their method of fabrication, and the controlled deposition of a contiguous catalytically active layer on the thus-formed core-shell electrocatalyst particles. This invention further relates to the use of these electrocatalysts in the electrodes of energy conversion devices such as fuel cells.
II. Background of the Related Art
A fuel cell is an electrochemical device capable of converting the chemical energy of a fuel and an oxidant into electrical energy. A standard fuel cell is comprised of an anode and cathode separated by a conducting electrolyte which electrically insulates the electrodes yet permits the flow of ions between them. The fuel cell operates by separating electrons and ions from the fuel at the anode and transporting the electrons through an external circuit to the cathode. The ions are concurrently transported through the electrolyte to the cathode where the oxidant is combined with the ions and electrons to form a waste product. An electrical circuit is completed by the concomitant flow of ions from the anode to cathode via the conducting electrolyte and the flow of electrons from the anode to the cathode via the external circuit.
The science and technology of fuel cells has received considerable attention, being the subject of numerous books and journal articles including, for example, “Fuel Cells and Their Applications,” by K. Kordesch and G. Simader, New York, N.Y.: VCH Publishers, Inc. (2001). Although there are various types of fuels and oxidants which may be used, the most significant is the hydrogen-oxygen system. In a hydrogen-oxygen fuel cell, hydrogen (H2) is supplied to the anode as the fuel where it dissociates into H+ ions and provides electrons to the external circuit. Oxygen (O2) supplied to the cathode undergoes a reduction reaction in which O2 combines with electrons from the external circuit and ions in the electrolyte to form H2O as a byproduct. The overall reaction pathways leading to oxidation at the anode and reduction at the cathode are strongly dependent on the materials used as the electrodes and the type of electrolyte.
Under standard operating conditions the H2 and O2 oxidation/reduction reactions proceed very slowly, if at all, requiring elevated temperatures and/or high electrode potentials to proceed. Reaction kinetics at the electrodes may be accelerated by the use of noble metals such as platinum (Pt), palladium (Pd), ruthenium (Ru), and related noble metal-containing alloys. Electrodes formed of these materials function as electrocatalysts since they accelerate electrochemical reactions at electrode surfaces yet are not themselves consumed by the overall reaction. Further improvements have been attained by incorporating noble metal-containing particles or structures with reduced dimensions. A reduction to nanoscale dimensions yields a significant increase in the surface-to-volume ratio, producing a concomitant increase in the surface area available for reaction. Despite the performance improvements attainable with nanoscale electrocatalysts, successful commercialization of fuel cells requires still further increases in performance, stability, and cost efficiency.
Pt has been shown to be one of the best electrocatalysts, but its successful implementation in commercially available fuel cells is hindered by its extremely high cost, susceptibility to carbon monoxide (CO) poisoning, poor stability under cyclic loading, and the relatively slow kinetics of O2 reduction at the cathode. A variety of approaches have been employed in attempting to solve these problems. An example is U.S. Pat. No. 6,232,264 to Lukehart, et al. which discloses polymetallic nanoparticles such as platinum-palladium alloy nanoparticles for use as fuel cell electrocatalysts. Another example is U.S. Pat. No. 6,670,301 to Adzic, et al. which discloses a process for depositing a thin film of Pt on dispersed Ru nanoparticles supported on carbon (C) substrates. These approaches have resulted in electrocatalysts with reduced Pt loading and a higher tolerance for CO poisoning. Both of the aforementioned patents are incorporated by reference as if fully set forth in this specification.
Attempts to accelerate the oxidation reduction reaction (ORR) on Pt while simultaneously reducing Pt loading have been met with limited success. Recent approaches have utilized high surface area Pt or Pd nanoparticles supported by nanostructured carbon (Pt/C or Pd/C) as described, for example, in U.S. Pat. No. 6,815,391 to Xing, et al. which is incorporated by reference as if fully set forth in this specification. However, as an oxygen reduction catalyst, bulk Pt is still several times more active than Pt/C and Pd/C nanoparticle electrocatalysts. Despite the continued improvement attained with Pt-based electrocatalysts, successful implementation in commercial energy conversion devices such as fuel cells requires still further increases in the catalytic activity while simultaneously improving long-term stability and reducing the amount of costly precious metals required.